of a Transit Circle . 
serves for strength and uniformity ; and to it is applied the 
clamp for elevation. That clamp is so made, as to allow the 
circle to run freely all round if you please ; not bearing at 
all against it, but supporting itself, and yet being easily re- 
moveable. It has no command over the circle whatever, 
when handled with care, excepting in the altitude of the te- 
lescope, by an adjusting screw when the clamp is set : and, 
as that screw has a milled head at each end, it is as conve- 
niently turned from the one as from the other side of the in- 
strument, to bring the horizontal wire to bisect the object. 
The telescope is of 2 inches aperture, and 33 focal length. 
The object-glass does not slide within the tube ; but screws 
into the end of a piece of false tube, of 4 inches length, which 
slides on the outside of the principal tube, and is fixed in its 
place, by 3 screws and collars running in grooves, when 
its distance from the wires is adjusted. In this way, one has 
the whole aperture of the tube ; and no greater length than 
is absolutely necessary for use ; which, in such an instrument, 
appeared to me to be an advantage. In some respects I find 
it so : yet, the hazard of disturbing the collimation, by touch- 
ing the outside of the tube, is an objection. 
The wires are not in one cell ; but in two distinct cells, with 
their faces towards eacli other. The perpendicular wires are 
5, at 35 seconds of time distance in the equator; and are ad- 
justable horizontally for collimation by a screw. The hori- 
zontal wires are 3, at about 1 5 minutes of a degree asunder ; 
placed so as just not to touch, but to pass clear of the other 
wires ; and they are adjustable in collimation by another 
screw peculiar to them. The two cells have each a power of 
MDCCXCIII. U 
